Metastable aluminum-titanium materials

ABSTRACT

This invention relates to the synthesis of a new generation of metastable aluminum-titanium (Al—Ti) alloys and the process of making them. The preparation method used is a combination process incorporating the advantages of conventional casting and spray techniques. The process is a low cost process. The aluminum-titanium materials made in this invention contain titanium in both the reacted and unreacted form. The results were confirmed using microstructural and x-ray diffraction studies. The presence of phases clearly indicate the metastable nature of these materials in accordance with the equilibrium phase diagram established for Al—Ti system. The Al—Ti materials can be made in the dimensions suitable for structural applications at ambient and elevated temperatures and as control materials for synthesis of more dilute equilibrium Al—Ti materials using conventional techniques such as casting.

This application is the national phases under 35 U.S.C. §371 of PCTInternational Application No. PCT/SG99/00069 which has an Internationalfiling date of Jul. 7, 1999, which designated the United States ofAmerica.

The present invention relates to metastable aluminium-titanium materialsand to a process for their manufacture.

In recent times the continuing and rapid advancement in technology hasled to the design and development of high performance devices capable ofoperating in increasingly hostile service conditions. From the existingtrend in the technological development it can be anticipated that theseverity of the service conditions that these devices will have towithstand will increase in the time to come. One of the importantcriterion to realize the continuing improvement in the efficiency ofthese devices is the development of materials that will be used to makethese devices and their ability to withstand increasingly hostileservice conditions. Aluminium alloys with titanium as the main alloyingelement are one such class of materials actively pursued for possibleapplication in relatively high temperature and high stress conditions.

The use of titanium in aluminium has been explored as an alloyingelement so as to synthesize high performance materials for advancedengineering applications. Processing techniques based on molten metals(such as conventional casting and spray atomization and deposition) andmetallic powders (such as powder metallurgy and mechanical alloying)have been investigated. Amongst these techniques, the synthesis ofaluminium-titanium materials can be carried out more cost effectively bythe methods based on molten metals. The studies conducted so far haveshown that the addition of titanium in the liquid aluminium leads to: a)an increase in the melting temperature of aluminium significantly and b)reaction between titanium and aluminium to form Al₃Ti through peritecticreaction. For example, addition of 10 weight percent of titanium inaluminium raises the melting point of aluminium from ˜660° C. to ˜1200°C. besides significantly increasing the chemical reactivity of theresultant molten Al—Ti mixture. This necessitates the use of moreexpensive and specialised furnaces (such as induction furnace) capableof heating to higher temperatures and crucibles made of highly inertmaterials thus increasing the overall cost of synthesis material. Theinteraction between titanium and aluminium in the molten conditionresults in either a solid solution of titanium in aluminium (with verylimited solid solubility) or the formation of Al₃Ti through peritecticreaction or primary solidification. The microstructures ofaluminium-titanium synthesized using conventional casting with slowcooling rate and spray atomization and deposition with reasonably highcooling rates (˜10³⁻⁴ K/s) revealed the as-expected existence of Al₃Tiintermetallic phase and equilibrium/extended solid solubility oftitanium in aluminium. Neither of these techniques has shown thecapability of synthesizing aluminium-titanium materials at temperatureslower than that exhibited by equilibrium Al—Ti phase diagram and inretaining the titanium as titanium in its elemental form followingsolidification by controlling the reaction between the molten aluminiumand titanium.

We have now found a process for producing metastable aluminium-titaniummaterials which may be successfully produced at temperatures lower thanthose required by the equilibrium Al—Ti phase diagram. The metastablealuminium-titanium materials have been produced by controlling thereaction of the titanium with the molten aluminium in order to retain aproportion of the titanium in its elemental form.

According to the present invention there is provided a process for themanufacture of a metastable aluminium-titanium material comprising thesteps of:

i) melting aluminium in a crucible;

ii) mixing solid particulate titanium with the molten aluminium; and

iii) disintegrating or spraying the molten mixture on a metallicsubstrate such that the molten mixture is deposited and solidified onthe metallic substrate,

and wherein at least a substantial portion of the titanium is retainedas elemental titanium after the molten mixture is solidified on themetallic substrate.

There is also provided a metastable aluminium-titanium materialmanufactured by the process described in the immediately precedingparagraph.

It will be understood that by the term “at least a substantial portionof the titanium is retained as elemental titanium” it is meant thatsufficient elemental titanium is present in the metastable titaniummaterial to strengthen the aluminium matrix. The presence of titanium inthe matrix advantageously serves to improve the mechanical behaviour ofthe aluminium at ambient and elevated temperatures as a result of itshigh melting point, strength and modulus properties. If not for thepresence of metastable titanium in the matrix and associated metastablestrengthening, the synthesis requirement for conventional Al—Ti alloyswith similar amounts of titanium in the form of precipitates or in solidsolution will necessitate a much more complex combination of meltingprocess (skull melting), furnaces (induction furnace), crucible(titanium) and superheating temperatures. As an example, synthesis of Alwith 10 wt. % Ti will require a superheat temperature of ˜1200° C. whencompared to 750° C. using the presently described process. It ispreferred that the elemental titanium retained in the metastablealuminium-titanium material is present in an amount ranging up to 20% byweight of the material.

It is preferred that the retained elemental titanium in the particulateform is uniformly distributed throughout the material. The uniformdistribution indicates a non-aligned and non-clustered (as much aspossible) distribution in three directions (an isotropic distribution).

In a preferred embodiment, the mixture of molten aluminium andparticulate titanium is poured or allowed to flow from the crucible, andis subsequently disintegrated using jets of inert gas, the spray fromthe disintegrated mixture being deposited and solidified on the metallicsubstrate.

The at least a substantial portion of the titanium may be retained aselemental titanium by controlling the exposure time of the titanium tothe molten aluminium. The period will be readily determined by simpleexperimentation having regard to the temperature of the aluminium towhich the titanium is exposed and the particle size of the solidparticulate titanium. The lower the temperature of the molten aluminium,the longer the treatment period which the titanium may be exposed to thehot aluminium. The larger the particulate size of the solid particulatetitanium, the longer the treatment period which the titanium may beexposed to hot aluminium. The emphasis however is to advantageouslyminimize the exposure time of the particulate titanium to the moltenaluminium so as to avoid undesirable reactions between the two and toenhance the retention of the elemental titanium by aluminium followingsolidification.

The solid particulate titanium may alternatively or additionally betreated prior to mixing with the molten aluminium in order to increasethe period the titanium may be exposed to the hot aluminium and toenhance the retention of elemental titanium in aluminium followingsolidification. For example, the solid particulate titanium may betreated to produce an oxide coating thereupon so as to decrease thereactivity of the solid particulate titanium to the molten aluminium.Titanium powders may be preheated to temperatures in the range of from600° C. to 815° C., preferably about 650° C. for a period of at least 30minutes, preferably 1 hour in order to produce an oxide surface layer.

The present invention further provides a metastable aluminium-titaniummaterial, of which a substantial portion of the titanium comprises solidparticulate titanium which is substantially uniformly distributedthroughout the aluminium.

The metastable aluminium-titanium material comprises the presence ofnearly uniformly distributed elemental titanium particulates in anon-aligned and non-clustered form in all three directions, with minimalreaction with the aluminium based matrix. The materials do exhibit thepresence of finite amount of Al—Ti based phases, and minimal amount ofnon-interconnected porosity. The presence of Al—Ti based phases isadvantageously mostly confined to the near vicinity of the titaniumparticles.

Aluminium suitable for use as the matrix of the metastablealuminium-titanium materials include the aluminium based materialscontaining alloying additions such as copper, silicon, zinc, iron,magnesium either independently or in combination with each other.

Preferably the aluminium is treated prior to melting in order toeliminate surface impurities. A suitable method for treating thealuminium includes washing the aluminium with water and acetone.

In the process of the present invention the aluminium is melted in aninert crucible or other suitable container. The metal may, for example,be melted by resistance melting based techniques.

The molten aluminium is then held at a temperature for the blending ofthe solid particulate titanium. The superheat temperature is so selectedso as to ensure the complete melting and sufficient fluidity of themolten metal so that it can be stirred easily and effectively.

The solid particulate titanium for use in the present inventionpreferably include ones with purity levels ≧99%. For the purpose ofother than a binary Al—Ti system, the present methodology mayincorporate any other titanium based particles in the size rangescontaining average particle sizes of preferably <200 μm.

The solid particulate titanium may be combined with the molten metal byany convenient means. Typically the solid particulate ceramic materialmay be combined with the metallic metal by their additions whilestirring the molten aluminium. The stirring is preferably done using asuitably designed stirrer being stirred in the speed range of 450rpm-900 rpm and placed in the crucible below the surface of the melt.The stirrer design employed in the present study comprised a shafthaving a length of about 24 cm and two blades pitched at about 45° tothe vertical, and having a diameter of about 0.6 D, where D is thediameter of the melt at rest.

The mixed blend, following the addition of the titanium particles, isimmediately poured or allowed to flow from the crucible.

The mixed blend poured from the crucible is then preferablydisintegrated. The poured molten mixture is most preferablydisintegrated using jets of inert gas. Suitable inert gases for use indisintegrating the poured molten mixture include argon, and nitrogen.The jets of inert gas are advantageously aligned at 90° to the axis ofmolten metal stream for best results. As a result of disintegration thestream of mixed blend is converted into a form of spray with the averagedroplet/splat size of about 180 μm. The resultant disintegrated mixturethus obtained is subsequently deposited into a metallic substrate.Typical metals used for substrate include iron and copper basedmaterials. The process advantageously allows the substrate to be used atambient temperature thus enabling to minimize the cost of the process.The Al—Ti materials can be made in the dimensions suitable forstructural applications at ambient and elevated temperatures and ascontrol materials for synthesis of more dilute equilibrium Al—Timaterials using conventional techniques such as casting.

Preforms produced by the process of the present invention areadvantageously near fully dense and in a near final shape with thematrix having a fine grained equiaxed microstructure. The preforms maybe produced in near final product form requiring minimal amounts ofmachining.

The metastable aluminium-titanium material produced according to thepresent invention at temperatures of about 750° C. are significantlylower than that predicted by the equilibrium phase diagram. The meltingtemperature of aluminium containing 6 weight percentage of titaniumunder equilibrium conditions will approach closely to 1100° C. and willhence require even higher temperatures (at least a 50° C. superheating)for processing through conventional molten metal methods. The materialof the present invention retains titanium as elemental titanium in themicrostructure following the solidification of aluminium. Furthermore,the low level of porosity which may be achieved indicates thefeasibility of these methods to be used for near net shape synthesis.Finally, an increase in microhardness exhibited by aluminium-titaniummaterials synthesized using the present invention when compared to purealuminium indicates that the presence of titanium in the aluminiummatrix will favourably increase the mechanical properties of theresultant bulk aluminium-titanium material.

A more detailed description of the present invention is furtherdescribed by the following non-limiting examples and accompanyingdrawings in which:

FIG. 1 Optical micrograph showing the presence of elemental titanium andthe interfacial reaction zone in aluminium-titanium material synthesizedusing Method A.

FIG. 2 Scanning Electron micrograph showing the presence of elementaltitanium and a very narrow interfacial reaction zone inaluminium-titanium material synthesized using Method B.

FIG. 3 EDAX mapping showing the evidence of presence of titanium andsignificant interfacial reaction zone in the case of aluminium-titaniummaterial using Method A.

FIG. 4 EDAX mapping showing the evidence of presence of titanium and avery narrow interfacial reaction zone in the case of aluminium-titaniummaterial synthesized using Method B.

EXAMPLES Method A: Limiting Reaction Time Between Molten Aluminium andTitanium Powder

The synthesis methodology of metastable aluminium-titanium materialusing Method A involved the following steps. Rectangular pieces ofaluminium were cut and subsequently washed using water and acetone toremove the surface impurities. After weighing, the cleaned pieces wereplaced in graphite crucible and superheated to 750° C. Titanium powdersequivalent to 6 weight percent were added into the molten aluminium meltstirred using zirconia coated stirrer at 465 rpm. The total additiontime of titanium powders was limited to not more than 3 minutes. Theresultant slurry thus obtained in the crucible was allowed to flow intoa 10 mm diameter stream through a centrally drilled hole in the crucibleand was disintegrated using argon gas jets at a distance of ˜255 mm fromthe pouring point and subsequently deposited onto a metallic substratelocated at a distance of 715 mm.

Method B: Modifying the Surface Characteristics of the Titanium Powders

The synthesis methodology of metastable aluminium-titanium materialusing Method B involved the following steps. Rectangular pieces ofaluminium were cut and subsequently washed using water and acetone toremove the surface impurities. After weighing, the cleaned pieces wereplaced in graphite crucible and superheated to 750° C. Titanium powdersequivalent to 6 weight percent were heat treated at 650° C. for 1 hourin order to produce a surface oxide layer [6] and were subsequentlyadded into the molten aluminium melt stirred using zirconia coatedstirrer at 465 rpm. The confirmation of the formation of oxide layer onthe titanium powders were made using the x-ray diffraction technique.The total addition time of titanium powders was limited to not more than3 minutes. The resultant slurry thus obtained in the crucible wasallowed to flow into a 10 mm diameter stream through a centrally drilledhole in the crucible and was disintegrated using argon gas jets at adistance of ˜255 mm from the pouring point and subsequently depositedonto a metallic substrate located at a distance of 715 mm.

For comparison purposes aluminium was also synthesized using theprocessing parameters similar to those used in Methods A and B.

Metastable aluminium-titanium materials synthesized in the present studywere characterized in terms of presence of elemental titanium,interfacial reactivity between aluminium matrix and titanium powders,porosity and microhardness of the metallic matrix. The presence ofelemental titanium was confirmed using x-ray diffraction technique (seeTable 1) and optical microscopy (see FIG. 1) which showsaluminium-titanium material produced using Method A. FIG. 2 shows ascanning electron micrograph of the aluminium-titanium material producedusing Method B. The extent of interfacial reactivity was determined byx-ray area mapping using EDAX (see FIGS. 3-Method A and 4-Method B);porosity was determined using image analysis (see Table 2) and themicrohardness measurements were made using an automated MatsuzawaDigital Microhardness Tester with a pryamidial diamond indenter using anindentation load of 50 g and a loading speed of 50 μm/s (see Table 2).

TABLE 1 Results of X-ray diffraction showing the presence of elementaltitanium and other selected phases in the case of aluminium-titaniummaterials synthesized using Methods A and B. Number of matching peaksPhases Pure Aluminium Al—Ti (Method A) Al—Ti (Method B) Al 5 5 5 Ti — 57 AlTi — 6 5 Al₂Ti — 6 5 Al₃Ti — 5 2

TABLE 2 Results of porosity and microhardness measurements conducted onpure aluminium and metastable aluminium-titanium materials. Pure Al—TiProperty Aluminium (Method A) Al—Ti (Method B) Porosity (%) 1.1 0.3 3.2Microhardness (HV) 32.0 ± 0.9 36.8 ± 0.5 35.0 ± 1.4

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is to be understood that the inventionincludes all such variations and modifications which fall within itsspirit and scope. The invention also includes all the steps, features,compositions and compounds referred to or indicated in thisspecification, individually or collectively, and any and allcombinations of any two or more of said steps or features.

What is claimed is:
 1. A process for the manufacture of a metastablealuminium-titanium material comprising the steps of: i) meltingaluminium in a crucible; ii) mixing solid particulate titanium with themolten aluminium; and iii) disintegrating or spraying the molten mixtureon a metallic substrate such that the molten mixture is deposited andsolidified on the metallic substrate, and wherein at least a substantialportion of the titanium is retained as elemental titanium after themolten mixture is solidified on the metallic substrate.
 2. A processaccording to claim 1, wherein in step (iii) the mixture of moltenaluminium and particulate titanium is poured or allowed to flow from thecrucible, and is subsequently disintegrated using jets of inert gas, thespray from the disintegrated mixture being deposited and solidified onthe metallic substrate.
 3. A process according to claim 1, wherein theat least a substantial portion of the titanium is retained as elementaltitanium by controlling the exposure time of the titanium to the moltenaluminium.
 4. A process according to claim 1, wherein prior to mixingwith the molten aluminium, the solid particulate titanium is treated toproduce an oxide coating thereupon.
 5. A process according to claim 4,wherein the solid particulate titanium is preheated to temperatures inthe range of from 600° C. to 815° C. for a period of at least 30 minutesto produce a surface layer of minimal reactivity.
 6. A process accordingto claim 4, wherein the solid particulate titanium is preheated totemperatures in the range of from 600° C. to 815° C. for a period of atleast 30 minutes to produce an oxide surface layer of minimalreactivity.
 7. A process according to claim 1 wherein, prior to melting,the aluntinium is treated to minimise surface impurities.
 8. A processaccording to claim 7, wherein said treatment comprises washing thealuminium with water and acetone.
 9. A process according to claim 1,wherein said aluminium is melted by a resistance melting basedtechnique.
 10. A process according to claim 1, wherein the metastablealuminium-titanium material contains a further component selected fromthe group consisting of copper, silicon, zinc, iron, magnesium andcombinations thereof.
 11. A process according to claim 1 wherein thesolid particulate titanium comprises titanium based particles having anaverage particle size of <200 μm.
 12. A metastable aluminum-titaniummaterial produced by a method as defined in claim 1, said particulatetitanium having an oxide layer, wherein said particulate titaniumretained in said aluminum-titanium material is present in an amount ofup to 20% by weight of the material, said particulate titanium having apurity of 99% or greater, and being uniformly distributed, non-aligned,and substantially non-clustered in each of three dimensions.
 13. Apreform produced by a process according to claim 1, wherein the preformcomprises said aluminum-titanium material, said aluminum-titaniummaterial having a grained equiaxed microstructure, wherein elementaltitanium retained in said aluminum-titanium material is present in anamount of up to 20% by weight of the material, and is uniformlydistributed, non-aligned, and substantially non-clustered in each ofthree dimensions.
 14. A metastable aluminum-titanium material producedby a process according to claim 1, comprising solid elementalparticulate titanium having an oxide layer thereon, said elementalparticulate titanium being present in an amount of up to 20% by weightof the material, and being substantially uniformly distributed,non-aligned, and substantially non-clustered in each of three dimensionsthroughout the aluminum.